276 Dr. J. D. Dana on the Origin of 



tous submarine slopes, the ' Challenger ' soundings give definite 

 facts as to one case. They leave undisturbed the previously- 

 reported cases of like steepness at greater depths : for 

 example, the sounding of Captain Fitzroy at Keeling atoll 

 (while Darwin was there), 2200 yards from the breakers, in 

 which no bottom was found at a depth of 1200 fathoms, but 

 the line was partly cut at a depth between 500 and 600 fathoms ; 

 the sounding by the Wilkes Expedition off Clermont Tonnere 

 (Paumotu Archipelago), where the lead, brought up an instant 

 at 350 fathoms and then dropped off again, descended to 600 

 fathoms without reaching bottom, and came up bruised, with 

 small pieces of white and red coral attached ; a sounding by 

 the same Expedition, a " cable's length " from Ahii, in which 

 the lead struck a ledge of rock at 150 fathoms and brought 

 up finally at 300 fathoms *. All the older soundings need to 

 be repeated ; but there must be enough truth in those quoted 

 to warrant the remark that the force of Darwin's argument 

 for subsidence from the steepness of the submarine slopes 

 about atolls is not weakened by the ' Challenger ' results. 



d. But the chief interest of the i Challenger ' soundings con- 

 sists in their affording " direct " proof, " positive " proof, of 

 much subsidence ; a kind of proof that subsidence sinks out of 

 sight, and w r hich soundings may yet make available in many 

 similar cases. 



That belt of coarse debris — including "masses 20 to 30 

 feet " long — was found over the steeply sloping bottom at 

 depths between 240 and 600 feet. These depths are far 

 below the limit of forcible wave-action. They are depths 

 where the waters, however disturbed above by storms, have 

 no rending and lifting power, even when the bottom is 

 gradually shelving ; depths, in this special case, against a 

 slope which for 100 yards is 75° in its upper part, and in no 

 part under 45°, the vertical fall being 360 feet in the 100 yards. 

 Strokes against the reef-rock thus submerged, and under such 

 conditions, would be extremely feeble. Waves advancing up a 

 coast, whether storm-driven waves or earthquake waves, do 

 little rock-rending below the depth to which they can bare 

 the bottom for a broadside plunge against the obstacle before 

 them, although the velocity gives them transporting pow r er 

 to a greater depth. It is the throw of an immense mass of 

 water against the front, with the velocity increased by the 

 tidal flow over a shelving bottom, — the rate sometimes 

 amounting, according to Stevenson, to 36 miles an hour or 

 52*8 feet a second, — together with the buoyant action of the 

 water, that produces the great effects, 



* Ibid. p. 55. 



